SA usually say that to make a few $, but in reality, it is not necessary, not after 1500 miles, but if dealer will do that under maintenance for free, then i will say it wouldn't hurt, but don't pay for it.

I have some SA keep telling me they can add fuel cleaner to the gas to clean the intake value for $50 (or whatever), what a load of crap, our cars are direct injected!

It is necessary as a function of time, because brake fluid is hygroscopic. BMW would not include it every 2 yrs. under free maintenance if it weren't necessary (they have their own interest in mind, not yours). Nothing like causing brake components to rust from the inside out, by avoiding the replacement of the fluid. It's not BMW alone, it's any car.

I just did my front brakes/rotors this past weekend along with changing the brake fluid. The difference in color was amazing. From dark ale color to the new clear yellowish tint. If it's free, have them do it.

My "Brake Fluid Service" light came on this morning. This car is 1 year, 8 months old and has 16,000 miles on it.

This seems strange to me since my 2008 E90 never did this even after 25,000 miles.

The brake fluid isn't a function of miles. The car could have 100 miles on it, however, still require a brake fluid flush.

The reason to change the brake fluid isn't due to the quality or color of the fluid itself, it's because over time moisture accumulates. While brake lines are sealed pretty well, over time very small amount of moisture contaminate the brake fluid.

At no point in time is this going to cause the brakes to fail... it only makes them less effective. In fact, MOST people drive cars for 5+ years without ever changing the brake fluid because the decline in brake effectiveness is so subtle over time. However, if you took on of those cars and changed the brake fluid, it would make a noticeable difference in the pedal feel during braking.

BMW recommends 2 year intervals because that's what they feel is the appropriate amount of time their standard fluid can perform at the level they consider standard for their car brand. The car will not blow up or even lose any reliability if you decide to forgo the service.

Does water propogate through the fluid evenly? i.e. is testing it in one spot (such as the master cylinder resevoir) valid for the fluid condition in all the calipers as well?

As mentioned brake fluid is hygroscopic absorbing moisture from the air
thus lowering the boiling point.
And yes it will eventually cause brake failure due to pockets of steam in the brake lines etc.
This can happen e.g. going down a mountain with long periods of brake
application.

Some intreresting things said here, but the main reason to change the fluid is as it retains moisture the water will not compress like true hydraulic fluid and the brakes will not be as effective due to not pushing as much hydraulic fluid thru the system.

I have never boiled brake fluid on a street car, only on the track. If you are seriously concerned about boiling your brake fluid go to DOT5 silicon racing fluid as that is hydroscopic and does not boil until over 500 degrees but has other issues and that's way most dedicated track cars change fluid often.

Also most DOT approved, in fact all of the ones that I can think of have corrosion inhibitors so to rust your brake parts internally you'd need to run a long time with a lot of water in the fluid and you'd really notice braking degradation I would hope first.

Do it every 2 years, it's not that costly and easy to do if you are somewhat handy.

Problem when you get water in the brake fluid is that the water will easily boil, and when it boils it places air bubbles into the system...air obviously is much more compressible than the fluids, and this is why it gets spongy feeling.

Brake fluid slowly absorbs water. It absorbs the water vapor present in air. The brake system is not sealed closed, so air is present in the master cylinder reservoir. If the system were sealed closed, as the brake pads wear and create space in the cylinders of the calipers, a vacuum would be created, which would eventually lock the system and make it not work.

Water in the fluid lowers the maximum boiling point of the fluid. When the fluid "boils" is now contains brake fluid in a gaseous state (not a liquid state - remember states of change: gas-liquid-solid - from high-school chemistry?). Once in a gaseous state, the fluid becomes compressible and does not transfer force from the master cylinder to the calipers as well as fluid in a liquid state.

And by the way to the OP, there is no "filter" in the brake system to change. Brake fluid is not like engine oil.